Physical magnitude

In physics one physical magnitude (or physical amount) is a physical property that we can express numerically, that is, it is a property or attribute that we can to size and express by a number. For example, length, since we can measure the length of an object and express it through a number.

What is a physical quantity? It is a physical property that can be quantified.

Beauty, love or human qualities, like goodness, are not physical magnitudes, we cannot measure it. We cannot speak of a beauty whose value is 4 or a love of 10, nor do we have measuring instruments that allow us to know how many times one person is more beautiful than another or measure how much love this person feels for another. However, if we can say that it has a height of 1.60 meters.

Physical magnitude - measure

Types of physical quantities:

There are two types of physical quantities: scalar and vector.

Scalar physical magnitude:

Scalar magnitudes are those that we can represent by means of a number followed by the respective unit. For example, length, mass, temperature, volume, time, among others.

Vector physical magnitude:

Vector quantities are all those that, to be able to fully represent them, not only the numerical value and its respective unit of measurement are enough, we also need to specify its direction and direction. For example, speed, acceleration, force, among others. We express vector magnitudes through vectors.

We can measure a large number of physical quantities: mass, length, time, temperature, amount of substance, electric current, light intensity, force, speed, density, among others. However, many of these are related to each other, for example, speed with length and time. For this reason, seven magnitudes, called fundamentals, have been established, from which all others can be defined (derived magnitudes); In addition, establish standard units for each of them.

Fundamental physical magnitudes

As mentioned above, the fundamental quantities are those that, thanks to their combination, give rise to the derived physical quantities and that, in principle, can be determined by means of a direct measurement. There are seven fundamental physical quantities, which have a basic unit (called a fundamental or basic unit).

Types of fundamental physical quantities

Among the fundamental physical quantities, with their respective basic unit (defined in the International System of Units or SI), we have:

Physical Magnitude

Name of the unit

Unity symbol

Electric current intensityampA
Amount of substancemoleMol
Luminous intensitycandlecd

Length (l)

It is the distance between two points. For example, when you measure your height, you measure the distance between two ends, the sole of the foot and the top of your head. The fundamental unit of length is the meter.

Mass (m)

It is a characteristic property that is related to the number and class of the particles that make up a body. Its basic unit is the kilogram, kg.

Time (t)

It is the period that elapses from a state A and the moment in which that state registers a variation (state B). Time allows events to be ordered in sequences, thereby establishing a past, a present and a future. Example, a person who walks changes the state each time he takes a step:

Physical magnitude - time

  • State A: moment when he starts walking.
  • State B: when taking the first step.
  • State C: when taking the second step, and so on.

The basic or fundamental unit of time is the second (s).

Temperature (T)

It is a magnitude referring to the common notions of heat and cold. It is related to the internal energy of a body; The latter, in turn, is related to the movement of the particles or atoms that make up the body. The more movement there is, the higher the temperature (the body is hotter). Temperature does not depend on the number of particles in an object and therefore does not depend on its size. The basic unit of temperature is the Kelvin.

Electric current intensity (I)

It is the amount of electric charge that a material passes through in a given time. Its fundamental unit is the ampere (A).

Amount of substance (n)

It is the quantity or number of fundamental entities (atoms, molecules, ions, electrons, other particles or specific groups of such particles) present in a body. The amount of substance is symbolized by n, while its fundamental unit, the mole, has no abbreviation, which is why it is written mole.

Luminous intensity

It can be defined as the flux of light that is emitted per unit of solid angle in a concrete and specific direction. Its basic unit is the candle, represented by the acronym cd.

Derived physical quantities

They are those that come from the fundamental magnitudes and are determined from them, using the appropriate expressions. These magnitudes also have their respective units, called derived units. For example, speed is a quantity obtained by dividing a length by time. In the International System, this combination of units is called the meter per second (m / s).

Some derived magnitudes

Physical MagnitudeName of the unitUnity symbol
Areasquare meter
Volumecubic meter
Densitykilogram per cubic meterkg / m³
Speed, speedmeter per secondm / s
Angular velocityradiate per secondrad / s
Accelerationmeter per second squaredm / s²
Angular accelerationradian per square meterrad / s²
Work, EnergyjouleJ
Electric chargecoulombC
Electric potential, induced voltagevoltV
Electric resistanceohmΩ
Magnetic fluxweberWb
Magnetic flux density, magnetic induction, magnetic polarizationteslaT